This invention relates generally to color television picture tubes and more particularly to such tubes which utilize an aperture or shadow mask structure for selectively directing an electron beam onto a fluorescent screen.
Various types of color tubes for television receivers have been suggested and fabricated. One of the more frequently used types includes means for selectively directing an electron beam or beams through the apertures in a metal mask structure to impinge upon selected areas of a luminescent screen, which screen may comprise discrete phosphor dots disposed in a mosaic layer on the faceplate of a cathode-ray tube. Three such dots normally constitute an elementary phosphor triad, comprising a red, a green and a blue phosphor dot, the dots being tangent to each other and ideally disposed so that the phosphor dot centers are coincident with the apexes of an equilateral triangle. By directing an electron beam through a given mask aperture at a predetermined angle, a particular one of the sub-elemental phosphor dots in an elementary triad can be excited to give off energy at a particular wavelength, i.e., indicate a particular color. In this manner the three primary colors are reproduced, and simultaneous excitation of two or three dots in the same triad is effective to produce other hues.
A color television picture tube may utilize three separate electron guns, or by switching the beam from a single gun may, in effect, provide three separate electron beams; these beams are deflected in a well known manner to pass through the mask apertures and impinge on and excite certain of the phosphor dots. The three beams are considered as originating in a common plane, perpendicular to the axis of the tube, which plane is denominated the plane-of-color-centers. When one of the three electron beams is directed through a certain mask aperture to excite a particular phosphor dot, it is desirable that the circular beam landing area be centered on the particular phosphor dot.
During operation of the color television picture tube, electrons in the electron beams strike the shadow mask. The electrons which strike the shadow mask cause the temperature of the shadow mask to increase. As the shadow mask increases in temperature the metal of the shadow mask expands causing the shadow mask to "dome" forward towards the phosphor screen of the faceplate. Doming of the shadow mask during normal operation of the tube is sufficient to induce registration errors between the beam landing areas and the phosphor dots. That is, a beam landing area may be shifted so as to impinge an adjacent color phosphor dot, producing a color purity error, or to leave altogether the targeted dot, producing a white field non-uniformity or "leaving" error.
A number of patents reveal attempts to solve the problems of triad location error, triad size error and triad shape error by the use of an elliptical configuration for the phosphor elements and/or the apertures in the shadow mask. The above-mentioned patents are; U.S. Pat. No. 2,947,899 issued to Kaplan, U.S. Pat. No. 3,663,854 issued to Tsuneta et al., U.S. Pat. No. 3,581,136 issued to Staunton, U.S. Pat. No. 3,705,322 issued to Naruse et al., and U.S. Pat. No. 3,777,204 issued to Robbins et al. These patents, however, have not addressed themselves to the problem of radial misregistration between beam landing areas and phosphor elements due to doming during operation of a tube.
The novel phosphor screen and shadow mask of the present invention overcome the problem of radial registration errors caused by doming of the shadow mask. This invention has general applicability, but is most advantageously applied to a shadow mask type color television picture tube having a delta-type electron gun assembly or an in-line type electron gun assembly wherein the phosphor screen of the tube is of the dot type, as opposed to a line type.